To prevent Apple Scripts from opening the Apple Script Editor on Mac OSX login the script should be export to an application within Script Editor. The application (.app) should be added to OSX login items, not the script (.scpt).

To do this From the Script Editor menu, choose File -> Export and select “Application” from the File Format dropdown.

This is necessary because if you only save it as a script (.scpt file), when run at login it will simply open in AppleScript Editor.

We want this to be an automated process so that the functionality runs without our having to do a thing. As an application, the script functionality will run the same as if you opened it in the editor and clicked the “Run” button.